1. Field of the Invention
The present invention relates to a method of forming a microstructure, and more particularly to a method of a forming a microstructure on a surface of a binary optical element (BOE) such as a diffraction grating having a step shape in cross section. The present invention also relates to a method of producing an optical element having such a microstructure.
2. Description of the Related Art
In recent years, the BOE has been receiving attention as a technique for producing a high-precision diffractive optical element. BOEs are step-shaped diffractive optical elements approximating diffractive optical elements having a blazed shape in cross section. For example, a diffractive optical element 1 having a blazed shape in cross section shown in FIG. 17A may be approximated by a diffractive optical element 2 having a step structure as shown in FIG. 17B.
The surfaces of transmissive optical elements are generally covered with an antireflection film for suppressing reflection of light. In the case of refractive lenses, they have a smooth surface and thus it is easy to form an antireflection film. In contrast, the surface of BOEs is not smooth. A technique of forming an antireflection film on the non-smooth surface of a BOE is disclosed in a paper entitled xe2x80x9cAntireflection-coated diffractive optical elements fabricated by thin-film depositionxe2x80x9d (Pawloski and B. Kuhlow, Opt. Eng. 33(11), 3537-3546, (1994)).
In the method disclosed in this paper, an antireflective film 12 is formed by depositing a material m for forming an antireflective film using ion beam sputtering at a right angle from above onto a substrate 11 having a step structure, as shown in FIG. 18. When an antireflection film 12 is formed on an element having a microstructure such as a BOE, it is desirable that the antireflection film be formed, as shown in FIG. 18, only on step surfaces 11a perpendicular to incident light parallel to the optical axis.
Another antireflection technique is disclosed in a paper entitled xe2x80x9cThe optical properties of xe2x80x98moth eyexe2x80x99 antireflection surfacesxe2x80x9d ( S. J. Wilson and M. C. Hutley, Optica. Acta. Vol. 29, No. 7,993-1009(1982)). In this technique, a microstructure is formed on the surface of a BOE so that the refractive index in a region near the surface continuously varies in a direction perpendicular to the surface thereby achieving antireflection capability. More specifically, a resist film 32 is coated on a substrate 31, and the resist film 32 is exposed to argon or krypton ion laser beams L1 (with a wavelength, xcex, of 458 nm or 351 nm) interfering with each other in X and Y directions, as shown in FIG. 19A, thereby forming, as shown in FIG. 19B, micro projections 33 whereby antireflection capability is achieved.
Still another antireflection technique is disclosed in a paper entitled xe2x80x9cDiffractive phase elements based on two-dimensional artificial dielectricsxe2x80x9d (F. T. Chen and H. G. Craighead, Opt. Lett., Vol. 20, No2, 121-123 (1995)). In this technique, an aluminum film 42 with a thickness of 100 nm is first formed on a quartz substrate 41, and then a resist film 43 is coated on the surface of the aluminum film 42, as shown in FIG. 20A. The resist film 43 is then exposed to an electron beam with a diameter of 70 nm using an electron beam exposure technique. Thereafter, the resist film 43 is developed to obtain a pattern such as that shown in FIG. 20B. The aluminum film 42 is then etched by means of reactive ion etching (RIE) using the resist film 43 as a mask as shown in FIG. 20C. Thereafter, as shown in FIG. 20D, the quartz substrate 41 is etched using the aluminum film 42 and the resist film 43 as a mask. The aluminum film 42 and the resist film 43 are then removed. Thus, a pillar-shaped microstructure 44 having antireflection capability is obtained as shown in FIG. 20E.
However, when an antireflection film is formed on a micro step-structure such as a BOE using the sputtering technique shown in FIG. 18, the micro steps cause the resultant antireflection film to be nonuniform in thickness as shown in FIG. 21. Furthermore, the antireflection film 52 is also deposited on the side wall 51a of each step. Because the side wall is parallel to incident light, the film deposited on the side wall causes degradation in the antireflection capability.
Furthermore, in this antireflection technique using an antireflection film, it is required to select a proper film material having an optimum refractive index depending on the wavelength of light. When light has a wavelength shorter than 300 nm, the optical characteristics of most film materials are not good for such a short wavelength. More specifically, in such a short wavelength range, most film materials have large absorption indexes and cannot provide a large refractive index difference. Even when antireflection is achieved, the allowable wavelength range is narrow. Furthermore, no good film forming techniques for practical production are available. Besides, sufficiently high reliability is not achieved.
In the technique shown in FIG. 19, when a microstructure is produced by means of exposure to laser beams interfering with each other, there is a possibility that interference of laser beams occurs to an insufficient degree which results in nonuniformity in a resist pattern serving as an antireflection structure. The nonuniformity in the resist pattern results in degradation in antireflection capability. Furthermore, because the resist film used to form the antireflection structure is made of an organic material which absorbs light with a wavelength in a certain range, antireflection capability is achieved only in a limited wavelength range. The organic resist film also has problems with reliability and durability.
On the other hand, in the technique of forming an antireflection microstructure on the surface of a substrate by exposing a resist film to an electron beam and developing it as shown in FIG. 20, if the surface of the substrate, on which the resist film is formed, has a microstructure, then defocus occurs in the exposure process and thus the resultant resist pattern becomes poor in uniformity. As a result, the size of circular-shaped pillars or holes formed in the antireflection structure becomes nonuniform. Another problem of the electron beam exposure technique is that a long time is required to form a pattern over a large area, because exposure is performed using only a single beam. Thus, this technique is not suitable for mass production.
It is an object of the present invention to provide a method of forming a microstructure and a method of producing an optical element without encountering the problem or problems described above.
It is another object of the present invention to provide a technique of forming a microstructure on the surface of an optical element, at a desired location using the same material as that of the optical element thereby imparting high antireflection capability or reflection enhancement capability to the optical element.
According to an aspect of the present invention, there is provided a method of producing a microstructure, comprising the steps of: forming a mask on a surface of a substance, the mask including a nucleus or an island structure formed via nucleation in a process in which a thin film is formed; and etching the surface of the substance via the mask. According to another aspect of the present invention, there is provided a method of producing an optical element, comprising the steps of: forming a mask on a surface of a substrate, the mask including a nucleus or an island structure formed via nucleation in a process in which a thin film is formed; and forming a microstructure having antireflection capability or reflection enhancement capability by etching the surface of the substrate via the mask.